1. Field of the Invention
The invention relates to electromagnetic clutches and, more particularly, to a self-adjusting multiple friction disc clutch having a stationary magnetic field.
2. Brief Description of the Prior Art
Self-adjusting clutches are well known in the art; however, in most of these devices, the adjusting feature occurs when the operating space between the armature and the magnet body exceeds a pre-determined value. That is, for the adjustment to take effect, the clutch must be released and re-engaged.
Some prior art designs have attempted to automatically self-adjust for wear of the frictional surfaces without requiring the clutch to release and to be re-engaged. However, these clutches, in effect, adjusted the disengaged spacing between the frictional surfaces in order to compensate for wear; that is, no adjustment occurred in these devices until the main friction surfaces wore to the extent necessary to actuate the self-adjusting mechanism. Thus, by their very nature these clutches did not maintain a constant pressure on the disc pack since the magnetic engaging force varied over the life of the clutch.
Continuous self-adjusting electromagnetic clutches have been broadly disclosed in the art; however, the teachings of these clutches fail to recognize the problems associated with the multiple friction disc stationary magnetic field clutches, with regard to flux flow and wear compensation. For example, U.S. Pat. No. 3,994,379 by Miller, owned by the assignee of the present invention, discloses a self-adjusting electromagnetic clutch wherein the stationary magnetic body is separated from other magnetic components in the magnetic circuit and, therefore, teaches a radial air gap around the magnetic body. This radial air gap results in a loss of efficiency of the clutch because of the loss of magnetomotive force which does not contribute to the magnetic holding force, resulting in a lower torque output for a specific unit of generated flux. To understand the importance of having very low air gaps in the magnetic circuit, one has only to consider that the magnetizing force required to produce 65 K lines in air as compared to iron is about 3800:1. Therefore, any radial air gap greatly effects the efficiency in terms of torque output for a given size magnetic body and coil.
Other prior art such as U.S. Pat. No. 3,724,619 as well as No. 3,744,609 by Miller, owned by the assignee hereof, further disclose continuous self-adjusting features, but teach the use of a radial air gap resulting in a certain percentage of flux leakage in the magnetic flux circuit. Again, the overall efficiency of the clutch is greatly reduced as a result of the radial air gaps.